ORDER V. CETACEA.
(Whales, &c.)
It has been customary to place the animals of this Order at the conclusion of the series of Mammalia; some eminent zoologists, however, have preferred to fix them between the Carnivora and the Pachydermata, connected with the former through the Seals, and with the latter through the Manatees. This we believe to be their true position.
The Cetacea are exclusively aquatic: unlike the Seals, which occasionally crawl out of the water, and bask in the sun, upon the rocks, the Whales and Porpoises never leave the element in which they are born. Their external form is that of a fish, "in the horizontal elongation of the body, the rounded and smooth surface, the gradual attenuation of the extremities of the trunk, and the development of fins, and especially of the tail as means of progression."[1] We saw in the Seals, the fore paws reduced to mere flippers, by the shortening of the bones of the limb, and by the envelopment of the fingers in the common integument. In the Order before us, the transformation is still more complete; the bones of the arm are still more shortened and flattened; those of the fingers, imbedded in cellular tissue, are so concealed beneath a thick skin, that all external trace of—them disappears, there being even no claws to indicate their number. The limb has become a mere paddle or oar, to be worked in a medium of great density, and hence the shoulder blade is of remarkable breadth, and the other bones solid and powerful.
FIN OF DOLPHIN.
The hind limbs we observed in the Seals placed very far behind, and forming two fan-shaped oars, whose direction was backward. In the Whales these limbs are altogether wanting (or are represented by one or two small pelvic bones, isolated in the flesh); but they are replaced by a broad cartilaginous fin at the extremity of the body, placed not vertically, as in the Fishes, but horizontally. This organ is the chief instrument of motion, which is mainly performed by alternate strokes upward and downward, rendered effective by its immense muscular power. In some of the larger Whales this caudal fin is upwards of twenty feet wide, and contains a hundred superficial feet.
The head is joined to the trunk without any contraction, so that the neck seems to be altogether wanting; the seven vertebræ, however, that belong to this part of the skeleton in all Mammalia, are present, but are so narrow, and so soldered together (anchylosed) as to appear like a single bone of inconsiderable thickness. Several species have a perpendicular fin standing up from the back, but it is merely cartilaginous, and never supported by any bony processes from the spine.
The modification of the respiratory organs requisite for animals whose whole life is passed in the sea is not the least curious point in the economy of the Cetacea. Formed for breathing air alone, and therefore compelled to come to the surface at certain intervals, they yet remain immerged for periods which would prove fatal to any other air-breathing animals. ‘The Whales can remain upwards of an hour beneath the surface. The object of breathing being to renew the vital qualities of the blood, by the absorption of oxygen from the air, it is manifest that if more blood could be oxygenised at once than is wanted for immediate use, and the overplus deposited in a reservoir until required, respiration could be dispensed with for a while. Creative wisdom has obviated the difficulty by this contrivance. The exhausted blood which is returned by the veins, having been renewed by communication with the air in the lungs, is carried to the heart, whence only a part is carried away into the system, the remainder being received into a great irregular reservoir, consisting of a complicated series of arteries, which first lines a large portion of the interior of the chest, then insinuating itself between the ribs, forms a large tata outside of them near the spine, and also within the spinal tube, and even within the skull. The blood thus reserved is poured into the system as needed, and thus frequent recurrence to the atmosphere is dispensed with. The windpipe does not terminate, as in other mammals, in nostrils at the extremity of the muzzle, but in an orifice at the very summit of the head, which, as the animal rises obliquely, is the first part that emerges from the surface, so that the admission of air to the lungs takes place without needless effort, or exposure of the body. The orifice, or orifices, for there are sometimes two, are called blow-holes, the expulsion of the long-imprisoned and heated air being accompanied with considerable noise, and with the ejection of water or steam. Cuvier thus explains the latter circumstance. "Let us suppose the Cetacean to have taken into its mouth some water which it wishes to eject. It moves its tongue and jaws as if it were going to swallow it; but, closing the pharynzx, it torces the water to mount into the nasal passages, . . . until it raises the valve (between the nasal passage and two pouches or reservoirs), and distends the membranous pouches above. The water once received into these pouches can be retained there until the animal wishes to spout. For that purpose it closes the valve, to prevent the descent of the water again into the nasal passages below, and forcibly compresses the pouches by means of the fleshy expansions which cover them; thus compelled to escape by the narrow crescentic aperture or blow-hole, it is projected to a height corresponding with the force of the pressure."
As the Cetacea descend to unknown depths in the sea, where the pressure of the incumbent water must be immense, the opening of this passage into the lungs requires to be guarded by a valve of no ordinary power. It takes the form of a conical stopper, somewhat resembling the cork of a bottle, but of so dense a texture, and so perfectly adapted to the orifice, that every drop of water is excluded. So closely interwoven are the fibres of this valve, that it can scarcely be cut with a knife.
There is another part of the structure of these interesting animals which has relation to the immense pressure to which they are subjected at great depths. It is the coating of elastic fat in which the whole body is enclosed. When we consider that the pressure is sometimes upwards of a hundred and fifty times as great as that of the atmosphere, we wonder that it does not crush the animal, by causing the collapse of every internal cavity. To sustain this pressure, the body is enveloped in a mantle of very peculiar elasticity; the skin itself is greatly thickened, but, by an open texture of its interwoven fibres, it is made to contain in its structure, a thick layer of oil or blubber. "A soft wrapper of fat, though double the thickness to that usually found in the Cetacea, could not have resisted the superincumbent pressure; whereas by its being a modification of the skin, always firm and elastic, and, in this case, being never less than several inches, and sometimes between one and two feet thick, it operates like so much india-rubber, possessing a density and resistance, which, the more it is pressed, resists the more."[2]
Another important office is performed by this thick coat of superficial fat. The grand homes of the Whales are the icy oceans of the Polar regions, where warm-blooded animals, as all the Mammalia are, would require protection against the rapid abstraction of heat from the body. Fat being a slow conductor of caloric, the envelopment of the whole body in a thick "blanket," as it has been termed, of this substance, retains the generated heat, and keeps the animal warm at the lowest temperature. Being lighter than water, it also greatly contributes to the buoyancy of the body. A dead Whale floats, but the carcase, when stripped of the blubber, sinks with precipitation.
Thus the whole organization of the Cetacea and its perfect adaptation to a sphere of actions, and to habits, very different from those common to the Mammalia, displays very numerous, striking, and unexpected examples of the infinite wisdom and goodness with which the Almighty God has made all His works. And surely the observation of such displays, and the awakening of our praises to Him, should be the first objects of scientific studies.